Can prep, shoot and post each carry their own subtotal?

The client signs off the shoot budget and the post budget separately, so each phase needs a number that stands on its own inside one budget. Give each phase its own Section: every Section closes with a row repeating its name and its own total, and all of them roll up into the one budget Total in the sidebar footer. One budget then holds a shoot number and a post number that stay in step.

Set the phases up

Open the sidebar Sections tab.
Click the + in the tab header, tooltip Add Section. On an empty budget the panel reads No Sections and Add Sections to structure your Budget. with an Add Section button instead.
The new Section opens straight into rename mode with the placeholder Type section name. Type the phase name and press Enter.
Repeat for each phase, then drag the Sections into the order you want them read.
Drag your General Cost Item and Production Cost Item lines from the Insert tab into the right phase.

Each phase now carries its own number, and the sidebar footer Total is the sum of all of them.

Split a phase further

Sections nest, so post can hold its own sub-blocks for picture, sound and grade, each with a total of its own that rolls into the post number. A sub-Section is created the same way as a top-level one: click the + and then drag the new Section onto the Section you want it inside.

Good to know

A Section total includes everything nested inside it, at any depth.
Within any one Section, the sub-Sections list first and the loose Cost Items follow, so a phase that mixes both reads as sub-blocks, then individual lines.
Smart Subtotals, the second sub-tab of the Sections tab, cuts the budget by AddOn, by Label and by currency across the whole budget. Sections give you the per-phase numbers.
Collapsing a Section hides its lines and leaves the phase heading and total on screen, which suits talking a financier through phase level.
The Cost Overview block in an Offer Document lists one row per Section with its total, so the phase numbers reach the client in the same shape.